1. Field of Invention
The present invention relates to a method for producing ferrite hollow particles having a hollow structure formed by mutual sintering of ferrite powder.
2. Related Art
For example in an electromagnetic shielding material, it is being executed to disperse a ferrite powder in a lightweight material such as a plastic material, in order to achieve a lighter weight and an easier molding. Also for achieving a further lighter weight, there is already known an electromagnetic shielding material utilizing ferrite hollow particles.
For obtaining ferrite hollow particles, there has been employed, for example, a method of dispersing a resin powder in a slurry containing a ferrite powder to obtain a resin powder coated with the slurry containing the ferrite powder, heat treating such resin powder thereby removing the resin powder and forming a hollow interior in the particles as shown in Unexamined Japanese Patent Publication Hei. 10-182264, or a method of dispersing a resin powder in a solution containing a metal or a metal compound (for example a metal oxide or a metal hydroxide) for constituting a component of ferrite, such as iron oxide, thereby obtaining a resin powder coated with the solution containing the ferrite constituting component, and heat treating such resin powder thereby removing the resin powder and forming a hollow interior in the particles for example, Unexamined Japanese Patent Publication Hei. 7-237923.
However, in either of these methods, an insufficient dispersion state of the resin powder results in a mutual binding of the resin, whereby the obtained ferrite hollow particles tend to assume non-spherical distorted shapes and non-uniform particle sizes. Also since the resin powder is dispersed in the slurry containing the ferrite powder or the solution containing the ferrite component then taken out and dried, the amount of deposition of the ferrite powder or the ferrite component often becomes uneven or insufficient in many cases to lead to an uneven thickness in the ferrite shell obtained after the heat treatment, frequently providing thin-walled shells. Therefore, the electromagnetic shielding material prepared with such ferrite hollow particles may show an inferior strength because of a stress concentration on the particles of such distorted shapes or thin shells.